An device of the kind referred to has been described in the European patent application having the publication number 0 166 392. This device can theoretically provide a considerable amplification of the measurement range. However, for this advantage to be useful in practice the sensitivity of this known device needs to be increased considerably.
It is therefore the object of the invention to increase the sensitivity of a Doppler measuring device such as the one referred to--i.e., to increase the signal-to-noise ratio--so that relatively weak useful signals can be considered in the measurement of flow velocity.